The present invention relates generally to an improved piston for a magneto rheological vibration damper of a motor vehicle suspension system.
Suspension systems for motor vehicles typically include a vibration damper to reduce vibrations derived from such variables as rough road surfaces, potholes, and unbalanced tires. Vibration damping is achieved by a piston stroking inside the damper each time the suspension system is vibrated. The piston strokes inside the damper through a fluid that resists the stroking action. The resistance to the piston stroking reduces the amount of vibration transmitted throughout the vehicle. The damping properties of the damper have proven to be enhanced through the use of magneto rheological (MR) fluid inside the damper. The sheer resistance of the MR fluid is altered proportionally to the strength of a magnetic field emitted through the fluid. Thus, the damping properties can be modified according to the variables producing the vibration.
A typical piston design includes a sleeve that circumscribes a piston core. MR fluid flows through a passage formed between the sleeve and the core as the piston strokes inside the damper. The vibration damping is derived from the resistance to the stroking action of the piston generated from the MR fluid inside the fluid passage. However, when the MR fluid is subjected to temperature extremes, its viscosity is known to change. The viscosity of the MR fluid impacts the damping properties of the damper in addition to the sheer resistance of the MR fluid. Temperature related viscosity variations deviate the desired damping properties of the vibration damper.
Therefore, it would be desirable to reduce the impact temperature related viscosity variations of the MR fluid have on the damping properties of the vibration damper.
The present invention relates to a piston assembly for a magneto rheological (MR) fluid vibration damper for a motor vehicle suspension system. A piston assembly strokes inside a housing filled with MR fluid. The piston assembly includes a piston core affixed to the end of a piston shaft. The core has an outer wall and includes an electric coil coaxially aligned with the piston shaft. The coil is connected to an electric wire that is inserted through the piston shaft. The coil generates a magnetic field when receiving electric current through the electric wire.
A sleeve includes an inner wall circumscribing the piston core defining a fluid passage between the inner wall and the outer wall. MR fluid flows through the fluid passage when the piston strokes inside the damper. The sheer resistance of the MR fluid is increased relative to the strength of the magnetic field generated by the coil. Thus, the level of vibration damping is determined by the strength of the magnetic field. At least one channel is formed in at least one of the inner wall and the outer wall. The channels can take different shapes including arcuate, rectangular, and triangular to meet the needs of a given vibration damper. At the channel, the distance is increased between the sleeve and the core.
The increased distance between the inner wall and the outer wall at the channel reduces the flow resistance variations derived from temperature induced viscosity changes of the MR fluid. The damper provides more consistent and precise damping properties when the impact of temperature induced viscosity variations of the MR fluid is reduced.